A pneumatic tire typically includes a pair of axially separated inextensible beads. A circumferentially disposed bead filler apex extends radially outward from each respective bead. At least one carcass ply extends between the two beads. The carcass ply has axially opposite end portions, each of which is turned up around a respective bead and secured thereto. Tread rubber and sidewall rubber is located axially and radially outward, respectively, of the carcass ply.
The bead area is one part of the tire that contributes a substantial amount to the rolling resistance of the tire, due to cyclical flexure which also leads to heat buildup. Under conditions of severe operation, as with runflat and high performance tires, the flexure and heating in the bead region can be especially problematic, leading to separation of mutually adjacent components that have disparate properties, such as the respective moduli of elasticity. In particular, the ply turnup ends may be prone to separation from adjacent structural elements of the tire.
A conventional ply may be reinforced with materials such as nylon, polyester, rayon, and/or metal, which have much greater stiffness (i.e., modulus of elasticity) than the adjacent rubber compounds of which the bulk of the tire is made. The difference in elastic modulus of mutually adjacent tire elements may lead to separation when the tire is stressed and deformed during use.
A variety of structural design approaches have been used to control separation of tire elements in the bead regions of a tire. For example, one method has been to provide a “flipper” surrounding the bead and the bead filler. The flipper works as a spacer that keeps the ply from making direct contact with the inextensible beads, allowing some degree of relative motion between the ply, where it turns upward under the bead, and the respective beads. In this role as a spacer, a flipper may reduce disparities of strain on the ply and on the adjacent rubber components of the tire (e.g., the filler apex, the sidewall rubber, in the bead region, and the elastomeric portions of the ply itself).
The flipper may be made of a square woven cloth that is a textile in which each fiber, thread, or cord has a generally round cross-section. When a flipper is cured with a tire, the stiffness of the fibers/cords becomes essentially the same in any direction within the plane of the textile flipper.
In addition to the use of flippers as a means by which to reduce the tendency of a ply to separate, or as an alternative, another method that has been used involves the placement of “chippers.” A chipper is a circumferentially deployed metal or fabric layer that is disposed within the bead region in the portion of the tire where the bead fits onto the wheel rim. More specifically, the chipper lies inward of the wheel rim (i.e., toward the bead) and outward (i.e., radially outward, relative to the bead viewed in cross section) of the portion of the ply that turns upward around the bead. Chippers serve to stiffen, and increase the resistance to flexure of, the adjacent rubber material, which itself is typically adjacent to the turnup ply endings.